I made a kolsch in the fall using Wyeast kolsch yeast. I had a very vigorous ferment, but I can't really give you a good take on the beer because it turned out to be my worst batch. It turned out tasting yeasty. I'm kind of disappointed because I was curious to find out what a kolsch would taste like.

if it where me, i'd use the White Labs German Ale/Koelsch yeast for a Koelsch (which I have a few times and liked) or the Wyeast Koelsch. if i was to brew an alt, i'd go with Wyeast's German Ale yeast. i've used the WL German Ale/Koelsch for an alt, and would want to try the Wyeast. again, just my preference.

Did you guys know, that in Germany Koelsh and Alt are pretty much arch enemies. Koelsh drinker (people from Cologne) are making fun of the Alt (people near Duesseldorf) drinker and vice versa. I came across this when I did some Alt research. And my comment that Alt may simply be a dark Koelsh was not very well taken. But I also plan to have both styles as regulars in my brewing schedule.

With regards to the yeast. WY1007 and WY2565 are very similar to each other (according to their description) and I don't see a problem in using each one of them for both styles if you are looking for the convenience of using just one strain rather than authenticity. I doubt that there will be noticeable differences in taste.